Arc-transducers have been found useful in many types of sonar systems where it is desirable to obtain a resolution in the near field that is better than that which can be obtained in the far field from conventional types of transducer arrays. With an unfocused array, a resolution approximately equal to the antenna size is the maximum resolution obtainable. Conventionally focused transducer arrays are capable of achieving high resolution at a specific range and in a specific direction. However, an arc-transducer has the unique capability of focusing all of its energy along a straight line thereby making such arrays useful in towed side-looking systems.
Generally, an arc transducer comprises a plurality of individual transducer elements driven in phase and positioned along a circular arc. While a plurality of individual piezoelectric elements is preferred because of the relative ease of manufacture, the arc-transducer can be fabricated as a single unit having a circular arc configuration. The energy from an arc-transducer is focused along a line that is normal to the plane in which the circular arc lies.
In side-looking sonar applications, for example, the active face of the arc-transducer has a width of approximately one wavelength and is about 100 to 1000 wavelengths in length. Typically, the active face or surface of the transducer elements is oriented so that it is directed to a point on the focal line of the transducer. In side-looking sonar applications, this direction or slant range is normally at a depression angle of about 10.degree. to the horizontal. In side-looking sonar systems, a depression angle of about 10.degree. achieves a large bottom range for a given slant range.
The depth-of-field is the range of depths over which satisfactory definition can be obtained in a direction parallel to the bottom and the plane in which the transducer lies. This direction is normally the direction of tow of the transducer platform. The resolution of the system, on the other hand, is established by the beam width at the maximum range. Consequently, the useful depth-of-field is the range of depths over which the beam width is substantially equal to or better than the beam width on the focal line at the maximum bottom range. In the past systems, it has been deemed desirable to increase the distance between the minimum and the maximum bottom ranges for any given pulse period and any given slant range by making the radius of the arc-transducer 10 to 20 percent of the maximum slant range.
The past practice, however, has a number of undesirable effects. One disadvantage is that a very small depth-of-field results at all ranges which means that if the vehicle containing the transducer is not at precisely the proper distance above the bottom, the bottom area is not in focus, resulting in an image obtained by the system that is of poor quality. Furthermore, objects viewed on the bottom at long ranges have very long shadows, and, at maximum range, little energy is reflected back because of small grazing angles. Finally, the system is critical to any roll of the transducer platform.
Accordingly, it is an object of the present invention to provide an arc-transducer that not only provides a large depth-of-field, but also provides a maximum search volume for a sonar system.